Method and system for coolant flow control for a prime mover in a vehicle propulsion system

ABSTRACT

A vehicle propulsion system includes a prime mover having a coolant inlet and coolant outlet, a coolant flow controller having a flow control inlet in communication with the prime mover coolant outlet, and a flow control outlet in communication with the prime mover coolant inlet, and a controller that determines a coefficient based upon a power of the prime mover, and that provides a coolant flow command signal to the coolant flow controller based upon the power of the prime mover.

FIELD

The present disclosure relates to a method and system for coolant flow control for a prime mover in a vehicle propulsion system.

INTRODUCTION

This introduction generally presents the context of the disclosure. Work of the presently named inventors, to the extent it is described in this introduction, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against this disclosure.

Vehicle propulsion systems may control coolant flow for a prime mover based on a predetermined set of gain coefficients that are determined by operating the prime mover at steady state in a number of operating conditions and determining the optimum set of gain coefficients for each operating condition. For example, in an internal combustion engine having a flow of coolant controlled by a valve, the engine may be operated at a selected speed and load and the proportional and integral gain coefficients for a valve controller that adjusts the flow of coolant to the engine which results in an engine coolant outlet temperature that best follows a desired temperature may be determined. In this manner, the gain coefficients may be calibrated to each engine speed and load and a table may be populated which stores the calibrated gain coefficients for each of a set of engine speeds and loads.

In this calibration procedure, the gain coefficients may be optimized such that the response is not overdamped or underdamped. An overdamped system may result in a coolant flow temperature which is very slow to respond. An underdamped system may result in a coolant flow temperature which repeatedly overshoots and undershoots the target temperature and may generally be unstable. An underdamped system will tend to overwork the components in the coolant flow control system which may lead to excessive wear and reduced durability. Further, an underdamped coolant temperature control may lead to excessive thermal cycling which may have an adverse effect on the components of the coolant system.

Using this system, the higher the number of operating conditions for which gain coefficients are determined to provide optimum coolant flow control the better. However, the calibration workload that is required also necessarily increases with the number of operating conditions for which optimum gain coefficients are determined in this manner.

SUMMARY

In an exemplary aspect, a vehicle propulsion system includes a prime mover having a coolant inlet and coolant outlet, a coolant flow controller having a flow control inlet in communication with the prime mover coolant outlet, and a flow control outlet in communication with the prime mover coolant inlet, and a controller that determines a coefficient based upon a power of the prime mover, and that provides a coolant flow command signal to the coolant flow controller based upon the power of the prime mover.

In another exemplary aspect, the controller further determines a power of the prime mover.

In another exemplary aspect, the system further includes a coefficient storage in communication with the controller that stores a table of coefficients each corresponding to a power of the prime mover and wherein the controller determines the coefficient by looking up a coefficient from the table of coefficients that corresponds to a power of the prime mover.

In another exemplary aspect, the coolant flow controller includes a coolant flow control valve.

In another exemplary aspect, the coolant flow command signal includes a coolant flow control valve position command signal.

In another exemplary aspect, the coolant flow controller includes a coolant pump.

In another exemplary aspect, the system further includes a temperature sensor that generates a temperature signal representing a temperature associated with the prime mover.

In another exemplary aspect, the coolant flow command signal is further based upon a difference between the temperature signal and a predetermined target temperature.

In this manner, by selecting control system coefficients based upon a power of the prime mover, control over the temperature of a prime mover in a vehicle propulsion system is greatly improved, which, in turn, improves fuel economy, efficiency, performance, reliability, and durability, reducing actuator oscillations and temperature fluctuations, improving responsiveness, reducing thermal strain, while simultaneously significantly reducing the workload associated with a calibration process that determines the coefficients corresponding to the power of the prime mover.

Further areas of applicability of the present disclosure will become apparent from the detailed description provided below. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

The above features and advantages, and other features and advantages, of the present invention are readily apparent from the detailed description, including the claims, and exemplary embodiments when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of an exemplary embodiment of a thermal management system 10 for a vehicle propulsion system in accordance with the present disclosure;

FIG. 2 is a schematic illustration of another exemplary embodiment of a thermal management system 10 for a vehicle propulsion system in accordance with the present disclosure;

FIG. 3 is a graph illustrating a correlation between engine power and optimized temperature controller gains;

FIG. 4A is a graph illustrating the temperature response of a vehicle thermal management system;

FIG. 4B is a graph illustrating the temperature response of a vehicle thermal management system in accordance with an exemplary embodiment of the present disclosure; and

FIG. 5 is a flowchart of an exemplary method in accordance with the present disclosure.

DETAILED DESCRIPTION

The inventors discovered that the thermal dynamics of a thermal management system for an internal combustion engine vary according to operating conditions. The gain coefficients which may be optimum for a specific operating condition may result in either over-gained (oscillatory) or under-gained (slow) response at other operating conditions. Therefore, in order to optimally control the temperature of that engine, the gain coefficients which are used should also change in accordance with the change in operating conditions. As a result of this, monitoring and adapting based upon temperatures alone has not proven reliable for changing operating conditions. The challenge, however, was in determining which of the measurable operating conditions most closely correlate to the optimum gain coefficients. The inventors discovered that there is a strong correlation between the power that is produced by a prime mover, such as an internal combustion engine, and the gain of coefficients of a controller that have been optimized to accurately follow a target temperature.

FIG. 1 is a schematic illustration of an exemplary embodiment of a thermal management system 10 for a vehicle propulsion system in accordance with the present disclosure. The system 10 may include a prime mover 12, such as, for example, an internal combustion engine. The system 10 may further have a coolant control valve 14 and a temperature sensor 16. A coolant may be circulated through the engine 12 to and from a cooling subsystem 18, such as, for example a heat exchanger. A controller 20 such as, for example, an engine control module, having one or more lookup tables 20 a stored in an associated non-volatile memory receives a temperature signal from the temperature sensor 16 and a signal, P, representing a power of the prime mover 12, to select one or more coefficients stored in the lookup table 20 a to generate a command signal to control the coolant control valve 14. The power signal, P, may be provided independently or separately from the controller 20. Alternatively, the power signal, P, may be calculated or otherwise determined by the controller 20 based upon other signals (not shown) which may be indicative of the power of the prime mover 12. The controller 20 may control the position of the valve 14 which, in turn, controls the flow of coolant through the prime mover 12.

FIG. 2 is a schematic illustration of another exemplary embodiment of a thermal management system 100 for a vehicle propulsion system in accordance with the present disclosure. The thermal management system 100 includes a prime mover 102, such as, for example, an internal combustion engine, a motor, a battery or the like without limitation which may provide a source for power to propel a vehicle in which the vehicle propulsion system resides. The exemplary embodiment schematically illustrated in FIG. 1 includes a prime mover 102 which includes a split cooling system. A split cooling system features separate cooling paths for the engine block 104, the head 106, and the exhaust manifold 108, which may be integrated with the head 106. A split cooling system may provide many advantages such as, for example, selectively controlling which of the block 104, the head 106, and/or the exhaust manifold 108 may be cooled. Typically, the head 106 has a much lower mass than the block 104 and, in an initial start-up condition, the head 106 may heat up much more quickly and require cooling while the engine block 104 may not need cooling while the block 104 continues to heat up to the desired operating temperature. In this manner, split cooling systems may provide multiple advantages by selectively controlling the cooling of each of the block 104, head 106, and exhaust manifold 108 which, in turn, may enable improved achievement of optimum oil temperatures, improved combustion conditions, faster warm up, improved fuel economy, efficiency, performance, and reduced emissions.

The thermal management system 100 may include a coolant pump 110 that provides a flow of coolant to the engine 102 via an engine coolant inlet path 112 which may be in fluid communication with each of the block 104, the head 106, and the exhaust manifold 108. The system 100 may also include a first coolant control valve 114 in fluid communication with a coolant outlet 116 of the block 104. The first coolant control valve 114 may be selectively operated to control the flow of coolant through the block 104. The system 100 may further include a second coolant control valve 118 in fluid communication with a coolant outlet 120 of the exhaust manifold 108. The second coolant control valve 118 may be selectively operated to control the flow of coolant through the exhaust manifold 108. Further, the system 110 may include a third coolant control valve 122 having an inlet that is in fluid communication with the outlet of the first coolant control valve 114, the second coolant control valve 118, and an outlet 124 of the head 106. In other words, the third coolant control valve 122 receives all of the coolant flow from the engine 102. The third coolant control valve 122 is selectively operable to determine the proportion of coolant flow that is provided to a bypass flow path 126 and a heat exchange flow path 128. The bypass flow path 126 provides a fluid communication path between the third coolant control valve 122 and the pump 110. The heat exchange flow path 128 includes a heat exchanger 130 that functions to exchange heat to and/or from the coolant flowing through the heat exchange flow path 128. In an exemplary embodiment, the heat exchanger 130 may be a radiator for releasing heat from the coolant to the ambient atmosphere. In other exemplary embodiments, the heat exchanger 130 may be an oil heat exchanger, a transmission heat exchanger, a heater core and/or the like without limitation.

The thermal management system 100 may further include a first temperature sensor 132 for sensing a first temperature associated with the block 104, a second temperature sensor 134 for sensing a second temperature associated with the head 106 and a third temperature sensor 136 for sensing a third temperature associated with the exhaust manifold 108. Additionally, the thermal management system 100 may include a controller (not shown for the sake of drawing simplicity and clarity) in communication with each of the temperature sensors 132, 134, and 136 and the valves 114, 118, and 122. The controller may be adapted to receive temperature signals from each of the temperature sensors 132, 134, and 136 and, based upon those signals, selectively operate each of the valves 114, 118, and 122 to control the flow of coolant through the engine 102.

Based upon each of the sensed temperatures, the controller may selectively operate associated valves in a manner that controls the flow of coolant through the split cooling system such that each of the sensed temperatures approach respective target temperatures. For example, the controller may control the block valve 114 such that the temperature from the block temperature sensor 132 follows a target temperature for the block 104. In an exemplary embodiment, the controller may operate the block valve 114 in accordance with the following:

dV=Kp×T _(error) +Ki×∫ _(t−1) ^(t) T _(error) dt   (1)

Where dV is the controller-determined change in the volume split between the block and the remainder of the engine, Kp is a proportional coefficient, T_(error) is the difference in temperature between the target temperature and the measured block temperature, and Ki is an integral coefficient. T_(error) may be determined according to:

T _(error) =T _(target) −T _(block)   (2)

Where T_(target) is the desired or targeted coolant temperature at the engine coolant inlet 120 and T_(block) is the actual coolant temperature at the engine coolant inlet 120.

In this manner, the thermal management system 100 may control the amount of heat removed from the engine 102 by controlling the relative proportions of those streams of coolant by adjusting the valves 114, 118, and 122. These valves may be adjusted with a closed loop controller that operates based upon an error or difference between a target temperatures and actual temperatures. However, the effectiveness of this thermal management system 100 is based upon proportional and integral coefficients which are determined by calibrating those coefficients for a specific set of conditions. As those conditions change from that specific set of conditions, the coefficients which correspond to optimized prime mover performance at a new set of conditions also change. Any reduction in temperature control of the prime mover as a result of a change in operating conditions may have adverse effects on the fuel efficiency, economy, durability, reliability, and performance of the prime mover. In an attempt to address the changing conditions, multiple different sets of coefficients may be determined which each correspond to a different set of operating conditions. For example, a set of coefficients may be determined through a calibration process for each of a set of different engine speeds and/or loads. However, determining these multiple different sets of coefficients requires a significant amount of calibration workload. Further, the operating conditions of a prime mover may vary widely and it becomes increasingly difficult to obtain a set of coefficients for each different set of operating conditions. Any increase in the resolution of the operating conditions for which coefficients are determined, requires a corresponding increase in calibration workload to obtain those coefficients.

In accordance with an exemplary embodiment of the present disclosure, the thermal management system may significantly improve the ability to accurately control the temperature of the prime mover for a wide range of varying operating conditions by referencing engine power. The inventors discovered that by comparing engine power across a wide range of operating conditions enables a scaling of the control system coefficients. In this manner, the control system and method accurately and reliably controls the prime mover temperature such that it more accurately follows a target temperature. In accordance with an exemplary embodiment, the control system coefficients may be continuously optimized across a wide variety of operating conditions based upon the prime mover power while simultaneously obviating a large calibration workload.

FIG. 3 is a graph 200 illustrating the correlation between engine power and optimized temperature controller gains. The horizontal axis 202 represents engine power and the vertical axis 204 represents gain coefficient value. The series of dots on the graph correspond to optimized gain coefficients 206 which were experimentally derived through a calibration process. The pattern of the calibration gains 206 on the graph may be described as following a best fit curve 208. The derivation for the equation describing that curve follows.

To further illustrate the relationship discovered by the inventors, the characteristics of an engine thermal management system may be described as:

Q=m _(coolant) C _(p,coolant)(T _(out) −T _(in))   (1)

Where Q is the heat rejection of the engine to coolant flowing through the engine, m_(coolant) is the mass flow rate of the coolant through the engine, C_(p,coolant) is the specific heat and constant pressure of the coolant, T_(out) is the temperature of the coolant at the engine coolant outlet, and T_(in) is the temperature of the coolant at the engine coolant inlet. The control system and method may form a closed loop on the temperature of the coolant at engine coolant outlet and Equation (1) may be rewritten as:

$\begin{matrix} {T_{out} = {\frac{Q}{m_{coolant}C_{P,{coolant}}} + T_{in}}} & (2) \end{matrix}$

With current engine thermal management systems, the mass flow rate of coolant through the engine may not be directly measurable. However, we may estimate the heat rejection of the engine, Q, as being approximately proportional to engine power under steady state conditions such that:

Q=F(P _(engine))≈C·P _(engine)   (3)

Substituting into equation (2) we arrive at:

$\begin{matrix} {T_{out} = {\frac{C \cdot P_{engine}}{m_{coolant}C_{p,{coolant}}} + T_{in}}} & (4) \end{matrix}$

Although the mass flow rate of coolant through the engine may not be directly measured, a valve in the engine thermal management system may control the amount of coolant flowing through the engine. For example, the operating position of a valve may determine the proportion of coolant that flows through the engine:

m _(coolant,engine)=m_(coolant,system)% Valve   (5)

Where m_(coolant,engine) is the mass flow rate of the coolant through the engine (corresponding to m_(coolaut) above), m_(coolant,system) is the mass flow rate of coolant flowing through the thermal management system that is in communication with the valve, and %Valve is the relative opening percentage of the valve. % Valve may be directly measured in some system and/or may correspond to a valve control signal that controls operation of the valve. Substituting into equation (4) then gives us:

$\begin{matrix} {T_{out} = {\frac{C \cdot P_{engine}}{m_{coolant}\mspace{11mu} \% \mspace{14mu} {Valve}\mspace{14mu} C_{p,{coolant}}} + T_{in}}} & (6) \end{matrix}$

A closed loop controller identifies the amount of change in % Valve that is needed to eliminate any difference between actual measured T_(out) and a desired target temperature engine coolant outlet. By taking the partial derivative of equation (6) with respect to coolant flow through the engine around a feedforward equilibrium point, this relationship may be characterized as:

$\begin{matrix} {\left. \frac{\partial T_{out}}{\partial\left( {\% \mspace{14mu} {Valve}} \right)} \right|_{ff} = {- \frac{C \cdot P_{engine}}{\left( {\% \mspace{11mu} {valve}_{ff}} \right)^{2}m_{{coolant}_{ff}}C_{p,{coolant}}}}} & (7) \end{matrix}$

Where % Valve_(ff) is the feedforward abstracted valve position and m_(coolant ff) is the feedforward engine flow at a given operating condition.

The higher the magnitude of this partial derivative, then the more sensitive engine temperature is to changing valve position and the smaller the change in valve position is needed to eliminate a given temperature error and the lower the gain that is necessary to provide stability while maintaining an acceptable response. In particular, we may expect that the ideal proportional gain a controller is proportional to the negative reciprocal of the partial derivative, that is:

$\begin{matrix} {K_{P,{ideal}} \propto {- \left( \frac{\partial T_{out}}{\partial\left( {\% \mspace{11mu} {Valve}} \right)} \right)^{- 1}}} & (8) \end{matrix}$

The inventors further understood that there is a relationship between the feedforward engine coolant flow, nominal heat rejection and therefore engine power, and target temperatures. At the feedforward equilibrium point we get:

C·P _(engine) =m _(coolant,ff)% Valve_(ff) C _(p,coolant)(T _(out,target) −T _(in,target))   (9)

Where T_(out,target) is the target temperature of the coolant at the engine coolant outlet and T_(in,target) is the target temperature of the coolant at the engine coolant inlet. Substituting this into the partial derivative equation (8), canceling out a few terms and rearranging to simplify provides:

$\begin{matrix} {\frac{\partial T_{out}}{\partial\left( {\% \mspace{14mu} {Valve}} \right)} = \frac{m_{{coolant}_{ff}}{C_{p,{coolant}}\left( {T_{{out},{target}} - T_{{in},{target}}} \right)}^{2}}{C \cdot P_{engine}}} & (10) \end{matrix}$

Therefore, by deriving and understanding this relationship, the inventors discovered that as engine power increase, then the gain coefficient for the temperature management controller should also increase.

With the above understanding in mind, a set of gain coefficients may be developed and/or calibrated which are selected based upon a prime mover power (i.e. a single variable) and stored in a table for use in a valve controller in a prime mover temperature control system based upon a single factor (i.e. prime mover power). In comparison to conventional calibration processes which develop a table of control system gain coefficients based upon different engine speeds and/or loads, the present disclosure greatly simplifies and reduces calibration workload and further improves the control over the prime mover temperature by reducing the complexity and identifying coefficients for different prime mover powers. This reduces the calibration task from two dimensions, to a single dimension.

In this manner, differing operating conditions which may have similar engine powers may have similar gain coefficients. Gain coefficients for a vehicle prime mover temperature control system may be scheduled based upon prime mover power.

While the previous description described a temperature management system that incorporate a valve that controls the proportion of coolant flow through the engine in comparison to the remaining portion of a larger system. It is understood that in another exemplary embodiment may be used with an internal combustion engine having a split cooling system where, for example, flow of coolant through the engine may be proportionally divided between a block, a head, and/or an integrated exhaust manifold and incorporating a block valve which proportions the amount of coolant flow through the block of the engine compared to the overall flow of coolant through engine as illustrated in, for example, FIG. 1. In this exemplary embodiment control over the temperature of the block may be accurately controlled by relating the mass flow of coolant through the block to the overall mass flow of coolant through the engine based on:

m_(coolant,block)=m_(coolant,engine)% Block   (11)

Where m_(coolant,block) is the mass flow of coolant through the block, m_(coolant,engine) is the mass flow of coolant through the engine, and %Block is the relative position of the block valve.

Further, while the previous description was provided with reference to an adjustment of a valve to control the flow of coolant through a thermal management system, it is to be understood that other components of such a system which varies the flow of coolant may also be similarly controlled. For example, rather than controlling the positon of a valve, the control system and method of present disclosure may be used to adjust the flow rate through a coolant pump.

Additionally, the control system and method of the present disclosure relies upon engine power to determine which of a set of gain coefficients may be used in a vehicle prime mover thermal management system. Vehicle propulsion systems typically calculate prime mover power which is then available throughout the vehicle propulsion system for many purposes, such as, for example, in a vehicle prime mover thermal management system in accordance with the present disclosure.

In an exemplary embodiment of the present disclosure, the controller may determine the power of the engine 102. Co-assigned U.S. Pat. No. 9,611,781, the disclosure of which is incorporated herein in its entirety, discloses a method for determining the power of an engine. To determine the power of the engine, the operating speed of the engine may be sensed by, for example, monitoring a signal from a crankshaft sensor, engine speed sensor and/or the like. An engine speed sensor may monitor the speed of the engine and return a monitored engine speed result to the controller. The controller may determine a maximum brake torque for the engine using, for example, the engine speed and one or more lookup tables and then calculate the engine power based on the maximum brake torque and engine speed. In general, the controller may multiply the maximum brake torque with the engine speed to calculate the engine power. The calculated engine power may then be used by the controller with the equations described above to modify the gain coefficients that are used by the controller to control the valves in the thermal management system.

There may be other systems and methods for determining prime mover power which may also be used with an exemplary embodiment of the present disclosure without limitation. For example, a rate of fuel flowing into an internal combustion engine, the heat capacity of that fuel, and the combustion efficiency of the internal combustion may also be relied upon for providing a prime mover power upon which to base a selection from a table of coefficients in a vehicle prime mover thermal management system. Alternatively, crank shaft power may be a good proxy for combustion power. And, in a preferred, exemplary embodiment engine power may be related to crankshaft power and used to determine which set of coefficients are used in a vehicle prime mover thermal management system to control prime mover temperature.

FIGS. 4A and 4B illustrate the improved response of an exemplary embodiment in accordance with the present disclosure in comparison to a conventional system. In FIG. 4A, line 300 represents coolant temperature and line 302 represents the position of a valve controlling a flow of coolant. FIG. 4A illustrates an underdamped response of the coolant temperature 300. The coefficients for the vehicle thermal management system which provides the results illustrated by FIG. 4A were obtained and are determined based upon engine speed and/or load. As the valve opens and closes and/or varies its position, the coolant temperature 300 oscillates and does not settle near a target temperature very quickly.

In stark contrast, FIG. 4B illustrates a great improvement in the response of a vehicle thermal management system in accordance with an exemplary embodiment of the present disclosure. The coolant temperature is represented by line 304 and the position of a valve controlling the flow of coolant is represented by line 306. Because the coefficients which are relied upon by the vehicle thermal management system are derived from and based upon engine power, the position of the valve 306 and the coolant temperature 304 settle much more quickly. The coolant temperature 304 approaches and much more quickly settles to the target temperature in FIG. 4B in comparison to FIG. 4A. Therefore, the wear and tear on the valve is significantly reduced as the valve does not move nearly as much as that shown in FIG. 4A. Further, the performance, fuel efficiency, economy, reliability, durability is greatly improved because the temperature controlled by the thermal management system is much better.

FIG. 5 illustrates a flowchart 500 of a method for controlling a temperature in a vehicle thermal management system in accordance with an exemplary embodiment of the present disclosure. The method starts at step 502 and continues to step 504. In step 504, the controller gets a power associated with a prime mover. For example, the controller may receive a predetermined power signal or, alternatively, the controller may calculate the power. The method then continues to step 506 where the controller then refers to a lookup table of coefficients which each correspond to a power and determine which of the coefficients correspond to the power of the prime mover. In other words, the controller looks up a coefficient or coefficients from a lookup table based upon the power of the prime mover. The method then continues to step 508 at which the controller receives a temperature signal and then continues to step 510. In step 510 the controller compares the received temperature signal to a predetermined target temperature to determine a temperature error signal. The method then continues to step 512 where the controller relies upon a control algorithm, the determined temperature error signal, and the coefficient (or coefficients) looked up in step 506 to calculate a command signal. In this manner, the command signal is based upon not only the temperature error, but also the power of the prime mover. The method then continues to step 514 where the command signal is received by a coolant flow controller such as, for example, a coolant flow valve, a coolant pump and/or the like which, is then responsive to control a flow of coolant. The method then continues to step 516 where the method ends.

This description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. 

What is claimed is:
 1. A method for controlling coolant flow for a prime mover in a vehicle propulsion system, the method comprising: determining a gain coefficient based upon a power of the prime mover; and controlling a coolant flow for the prime mover based upon the determined gain coefficient.
 2. The method of claim 1, further comprising determining a power of the prime mover.
 3. The method of claim 1, wherein determining a gain coefficient comprises looking up a gain coefficient in a table of gain coefficients based upon the power of the prime mover.
 4. The method of claim 1, wherein controlling a coolant flow for the prime mover comprises controlling a coolant flow control valve.
 5. The method of claim 4, wherein controlling a coolant flow control valve comprises controlling a position of a rotary valve.
 6. The method of claim 1, wherein controlling a coolant flow of the prime mover comprises controlling a coolant pump.
 7. The method of claim 1, further comprising sensing a temperature associated with the prime mover and wherein controlling a coolant flow for the prime mover is further based upon the sensed temperature.
 8. The method of claim 1, wherein controlling a coolant flow for the prime mover is based upon a difference between the sensed temperature and a predetermined target temperature.
 9. A vehicle propulsion system, the system comprising: a prime mover having a coolant inlet and a coolant outlet; a coolant flow controller having a flow control inlet in communication with the prime mover coolant outlet, and a flow control outlet in communication with the prime mover coolant inlet; and a controller that determines a coefficient based upon a power of the prime mover, and that provides a coolant flow command signal to the coolant flow controller based upon the power of the prime mover.
 10. The system of claim 9, wherein the controller further determines a power of the prime mover.
 11. The system of claim 9, further comprising a coefficient storage in communication with the controller that stores a table of coefficients each corresponding to a power of the prime mover and wherein the controller determines the coefficient by looking up a coefficient from the table of coefficients that corresponds to a power of the prime mover.
 12. The system of claim 9, wherein the coolant flow controller comprises a coolant flow control valve.
 13. The system of claim 12, wherein the coolant flow command signal comprises a coolant flow control valve position command signal.
 14. The system of claim 9, wherein the coolant flow controller comprises a coolant pump.
 15. The system of claim 9, further comprising a temperature sensor that generates a temperature signal representing a temperature associated with the prime mover.
 16. The system of claim 15, wherein the coolant flow command signal is further based upon a difference between the temperature signal and a predetermined target temperature. 